The invention relates to the writing of servo information on magnetic disk recording media.
The organization of information recorded on a magnetic disk as known to the prior art is shown in FIG. 1. The magnetic disk 10 is divided into a series of concentric rings 12 termed tracks. Each track is circumferentially divided into a series of sectors 13. Each track 12 is separated from each adjacent track 12 by a ring called a guard band 14 in which nothing is recorded. The guard bands 14, which alternate with the tracks 12, provide a safety area so that a read/write head writing to a track 12 may deviate from the exact center of the track 12 and not corrupt the data written on adjacent tracks 12.
When a read/write head is moved to a specified track 12, it determines its actual position relative to the center of the track 12, by locating servo blocks 16 and 18. The two servo blocks 16 and 18 are located between adjacent sectors 13. The servo blocks 16, 18 are positioned radially adjacent each other, straddling the track with which they are associated such that their common boundary lies at the center of the track. The outer servo block 16 has a different magnetic orientation than does the inner servo block 18. Each block therefore induces a different polarity signal in the read/write head as the block passes under the read/write head. When the head is positioned over the center of the track 12, both servo blocks 16, 18 induce an equal but opposite signal in the head. If however, the head is not positioned over the center of the track, for example, if the head is located too far toward the inner circumference of the track, the inner servo block 18, will induce a stronger signal in the recording head than will the outer servo block 16. The circuit which controls and determines the head position will detect this condition and move the head back toward the outer edge of the disk until the signals from both servo blocks 16, 18 are equal.
The formatting of the disk into tracks, sectors, guard bands, and servo blocks, as well as the recording of other servo information on the disk during the disk's manufacture is performed by a device called a servowriter. The servowriter positions a read/write head accurately and repeatably at specified radial distances from the disk center so that the formatting of all the disks will be the same. Typically, the accurate positioning of the head at certain distances from the center of the disk is accomplished through the use of optical encoders attached to the positioning mechanism.
However, as the number of tracks per inch is increased to increase the information density of the disk, resulting in narrower tracks and guardbands, a need arises for an increase in the accuracy with which the read/write head is radially positioned during the formatting of the disk.